Cats get heartworms too! Heartworms live and replicate in dogs and are transmitted from an infected dog through mosquitoes to other dogs and in some events cats. The heartworm lifecycle requires a dog to live in and make baby heartworms, microfilaria. The cat is not the ideal host and the worm is unable to replicate in the cat. Most cats’ immune system kills larval stages of heartworms before they can migrate to the heart.


However, some worms are able to evade the body’s immune system and can migrate to the heart and lungs. There are usually fewer worms in cats than in dogs, but cats can have more side-effects. The most common signs are wheezing, coughing, asthmatic tendencies, and difficulty breathing. Some cats’ first sign is a seizure, severe lethargy, collapse or sudden death.

The most frustrating part of heartworm disease in cats is that there is no approved treatment for the cat. The heartworm injection given to dogs to kill the adult heartworm is not safe to use in cats. The best option for cats is to be on heartworm prevention year-round, even if indoors only. If a cat is showing progression of signs such as asthma, or having difficulty breathing it should be seen by a veterinarian. Testing for heartworms is more difficult than testing in dogs. The sample is usually needed to be sent out to confirm the presence of heartworms in cats. Cats are also more likely to test negative when they are actually positive than dogs. Because there is no treatment approved in cats, supportive care is the only option. The goals of therapy are to reduce the inflammation and heart damage done by the worms, and prevent new infection. This is not an ideal scenario because heartworms can live for years inside the heart and lungs of a cat. The whole time the worm is living in the heart and lungs, it is causing irreversible damage to the respiratory and circulatory tracts.

By far, the best option is to keep a cat on prevention year-round even if they are indoors only. It is important to remember that there are indoor only cats but there is no such thing as indoor only mosquitoes.


Todd Smith, DVM